The present invention generally relates to controlling the heating of architectural panels and, more particularly, to a heated architectural panel system and a method for controlling the temperature of heated windows formed from the architectural panels.
U.S. Publication No. 2003-0127452 to Gerhardinger et al. (which is incorporated herein by reference, hereinafter '452 Publication) teaches the use of various electrical control systems for various types of heated panels.
U.S. Pat. No. 6,303,911 to Welch Jr. utilizes an electrically real resistor R1 in series with a thin film resistive coating R2 on a thin film glass heater that is used to heat a small liquid crystal display (LCD). In this LCD application, it appears that a control circuit provides a direct current (DC) supply voltage Vsupp to the series resistor R1 and a thin film coating, which is assumed to have a known constant reference resistance R0 at a corresponding reference temperature T0. Through the use of the modeling technique R2=R1(Vsupp/V1−1) and T2=T0+(R2−R0)/a, where V1 is the voltage drop across the series resistor R1 and “a” is defined to be a unique constant associated with particular thin film coating materials, the varying temperatures T2 of the LCD application are estimated from R2.
The LCD application appears to be suited for a printed circuit board (PCB), where the voltage Vsupp produces a DC current, which is relatively small (e.g., milliamps or less). The series resistor R1, which forms a voltage divider with the thin film coating, appears to be physically small in size (possibly using or requiring in the order of a few square inches of area) and appears to be close in proximity to the LCD (e.g., no more than several inches apart).
In contrast to the LCD application, an architectural panel requires a much larger amount of heat that is supplied by alternating currents (AC) from several amps on up, wherein an impedance (that would include the series resistor R1) would need to be large in size and, therefore, not desirable for an architectural application due to the generation of wasted heat. Additionally, it would not be advisable to provide high alternating currents in close proximity to low LCD segment currents, due, for example, to electromagnetic interference (EMI) and radio frequency interference (RFI).
In addition to being a performance inhibitor, the utilization of the series resistor can present potential safety problems by moving the reference point voltage of the glass window above that of AC neutral (i.e., essentially above ground potential), which can result in multiple paths to ground (commonly known as ground loops).
Also, building window heaters are physically large in size (e.g., many square feet), and the windows, power sources, and control circuits are likely to be separated by long distances from one another (often by as much as 100 feet or more).
When conducting electricity, the coating on a vertically oriented heated architectural panel does not act as a single resistor. Because heat rises due to convection, the top of the heated architectural panel becomes warmer than the bottom and the center of the panel. Also, the temperature at the center of the panel tends to be higher than the temperature at the sides of the panel. Therefore, if the coating were assumed to be one resistor, then the top and center of the panel would be overheated, when compared to the lower and side portions of the panel, and the bottom and sides of the panel would be under-heated, when compared to the upper and center portions of the panel. Therefore, compensation for these non-uniform temperatures must be considered in the architectural application, whereas the small LCD application need not be concerned with such factors.
As a result of the large size and the long separation distances, the reference resistance R0 and, correspondingly, the reference temperature T0 can vary from window to window and under varying operating conditions. If the LCD application modeling were to be applied to known heated architectural panel installations, large transformers and AC/DC converters would be required. This practice, however, would result in wasting significant amounts of electrical power.
Due to ever increasing and high energy costs, proposed industry window energy standards will likely require higher energy efficiencies for heated window applications, which the use of the LCD application modeling techniques could not provide.
On the other hand, measuring the temperature of heated architectural windows, without the use of a sensor, while utilizing AC power to heat the windows, presents additional challenges. For example, in order to determine the resistance of the coating in an AC powered window, careful consideration needs to be given to line voltage fluctuations and transient surges. Otherwise, the resistance of the coating cannot be correctly determined.
In conjunction with the use of AC power and because the various parts of an architectural thin film heater circuit are distant from one another, consideration must also be given to EMI, RFI (in fact the heated window assembly can act as an antenna), robust electrical connections, lead wire voltage loss, shielding of wiring, and other factors.
As a result of these differences between the small scale LCD application and the large scale architectural applications, the modeling techniques of the LCD application do not effectively apply to architectural applications.
Thus, those skilled in the art continue to seek a solution to the problem of how to provide a better heated architectural panel system and a method for controlling the temperature of heated windows formed from architectural panels.